1. Field of the Invention
The present invention relates to a cross-polarization interference canceller (XPIC) using a fractionally-spaced adaptive filter for generating a compensation signal.
2. Description of the Related Art
In a radio transmission, a technique utilizing orthogonal polarization has been realized in order to elevate the utilization efficiency of frequencies, wherein signals are carried on two polarized waves orthogonal to each other (a vertically polarized wave (V) and a horizontally polarized wave (H)) having the same frequency.
In a system utilizing the orthogonal polarization, a high level of XPD (cross-polarization discrimination) is required to ensure stable transmission. Since the XPD sometimes deteriorates, for example, because of multipath fading in a microwave band or because of the presence of inclined raindrops in a frequency band higher than a semi-millimeter wave, the XPIC is introduced into the system in order to compensate for the deterioration of the XPD.
An adaptive transversal filter is normally used for shaping a received signal in the other polarization wave to thereby generate a compensation signal. The compensation signal is added to a main signal to compensate for the cross-polarization interference distortion. Coefficients of the adaptive transversal filter are controlled by correlating signals at tapped delay line nodes of the filter with an error signal in a compensated signal.
When a randomly varying phase difference occurs between two orthogonal polarization waves because of multipath feeding, etc., the compensation signal generator operating in a frequency of the transmission data cannot precisely compensate for the cross-polarization interference distortion. To overcome this problem, the introduction of a fractionally-spaced adaptive transversal filter having a fractionally-spaced tapped delay line and for updating the coefficients in a frequency more than two times the transmission data frequency, has been considered.
In a conventional compensation signal generator using the fractionally-spaced adaptive transversal filter, the error signal is updated only in the transmission data frequency. However, the signals at tapped delay line nodes used to calculate the coefficients are updated in the operating frequency of the filter because estimates of transmission data used to generate the error signal are obtained only in the transmission data frequency in a decision-directed adaptation. Therefore, in the conventional compensation signal generator, the adaptive filter applies an error value in the first half of a data interval to an error value in the second half of the data interval.
For this reason, the above compensation signal generator does not generate a precise compensation signal, particularly, in the second portions of data intervals. For example, in the extreme case of D/U=.infin. (i.e., in the case that cross-polarization interference does not occur), although the compensation signal generator must generate a compensation signal having a level of zero, the compensation signal output from the generator has a level of zero in the first half of the data intervals and has levels freely varying in the second half of the data intervals. This results because the compensated signal in the second half of the data intervals are not reflected in the correction of levels of the compensation signal. This phenomenum wherein levels of zero and levels freely varying alternately appear is referred to as the "skewer" phenomenum.